In a wireless base station of a communication system using cellular phones or the like, a high power amplifier (HPA), which amplifies and outputs a transmission signal, is arranged in a transmission processor. Generally, an HPA gives priority to amplification efficiency and thus has a low input-output characteristic linearity. That is, in an HPA, the input-output characteristics between an input signal and an output signal are non-linear distortion characteristics. Accordingly, when amplifying an input signal with an HPA having such input-output characteristics, the desired output signal may not be obtained due to distortion. As one distortion compensation method that compensates for such distortion, the following non-patent document 1 proposes a method (so-called digital predistortion: DPD) that compensates for distortion in the input-output characteristics of the amplifier by estimating a model expressing the input-output characteristics of an amplifier, generating an inverse model having characteristics inversed from the model through digital signal processing, and then adding the inverse model to an input signal (digital signal prior to conversion to an analog signal) for the amplifier. Non-patent documents 2 and 3 also propose high-efficiency amplification techniques.    Non-Patent Document 1: Lei Ding, “Digital predistortion of Power Amplifiers for Wireless Applications”, Georgia Institute of Technology, March 2004.    Non-Patent Document 2: Donald F. Kimball, et al., “High-Efficiency Envelope-Tracking W-CDMA Base-Station Amplifier Using GaN HFETs”, IEEE Transactions on Microwave Theory and Techniques, Vol. 54, NO. 11, November 2006.    Non-Patent Document 3: Feipeng Wang, et al., “Design of Wide-Bandwidth Envelope-Tracking Power Amplifiers for OFDM Applications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 53, NO. 4, April 2005.